O’Brien made 104 appearances for the Linnets since joining from Cammell Laird in 2016, scoring 20 goals.
Manager Michael Ellison told the club’s official website: "What can I say about Danny O'Brien? he's been absolutely fantastic for us since he first stepped through the door.
"He was one of the players, who stayed when I first got the job and helped stabilise the club during a difficult period. He was also a massive part of the title-winning side and he will be sorely missed at the Millbank.
"I did try to persuade Danny to stay but I do understand his reasons for leaving. I'd like to wish him all the very best where ever he decides to play his football next season and he will always be welcome back at Linnets.
"Top player, top lad. good luck Dan."
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